I have a win2k box at work that's running with 64Mb RAM!

Not fast, but it runs.
128 is actually fine unless you are doing big database searches. 256 should be plenty for win2k.

Any CPU over 300MHz should be fine for simple tasks in win2k. I assume the database is on the LAN somewhere, so each client will not be running big database queries?
If it was XP, though, that needs at least 512MB RAM to run nice.
Oh, and make sure all hardware in the PC's is supported by win2k before you start. Could save some hassles.
Link to HCL for win2k.
Cheers
Mick